Presenting Pump Power Peculiarities, Playing With Pumps And Pipes

Pump power exhibit at the Damen Dredging Experience
Pump power exhibit at the Damen Dredging Experience

Hej kära läsare, jag vill ta dig till ett land långt borta, för länge sedan. Min älskade Sverige.

In 1996, I started my graduation with Skanska1 in Sweden. They had a project to clean up a lake2 with an auger dredge. The auger was not performing and they asked the Delft University of Technology to investigate the problem and write a report with my solution. Off, I went to Växjö and spent a year on a dredge. During my reconnaissance of the project in the first week, I noticed that the flow in the pipe line was very slow and the motor was hardly working at full speed. As an innocent student, I asked where they were pumping the material to. ‘Oh, through 7 kilometre of pipe and 30 meter up into the hills.’ They were lucky it was such a fluid material and did not settle at such a low velocity. I then proposed they should buy a booster station to increase production3, as I could not see anything wrong with the auger. ‘No, no. It has to be the auger and the engine is strong enough; you see, there is no power required.’

Clean up project at Södra Bergundasjön near Växjö
Clean up project at Södra Bergundasjön near Växjö

That was the first time I saw the slow flow fallacy at work. Intuitively you would expect, that a long pipeline would require more power to transport the mixture than a short pipe line. This is exactly what this exhibit is trying to visualise. Water can be pumped through either the short or long pipe. From the lines on the tank wall, you can read that the output velocity of the fluid is about 1.5 m/s. In the vertical pipe, the delivery pressure is indicated. Multiplying pipe velocity and fluid pressure results in the actual power in the pipe line. The pump has to provide this power, by converting electrical power to mechanical power and eventually fluid power. On the display at the left of the buttons, the consumed electrical power can be read.

Discharge capacity through the short pipe line
Discharge capacity through the short pipe line

When you select the short pipe line, you have to notice the higher flow velocity and the required power at the display. Switching over to the longer pipeline, you will notice a drop in velocity. Due to the longer pipe, the fluid experiences more resistance. For the same pressure, the flow will be lower. Consequently, the power consumption will be lower also! This is exactly according to the theory. A pump at a lower capacity will consume less power, even if the pressure rises slightly.

Discharge capacity through the long pipe line
Discharge capacity through the long pipe line

Off course, the delivered mixture will be less, on the longer line. You might increase the speed of the pump to have more pressure. And indeed, that would require more power. But there is a maximum speed on the pump drive. Same for a very short pipe. You might end up below the idle speed of the diesel engine. Be careful in your project layout that you do take into account this viable operating range for the length of the pipe line. A longer pipeline might require a booster station for increased production. Conversely, a shorter pipe line might be chosen with a smaller diameter for increased resistance and lower power consumption, while keeping the operating point of the dredge pump near the Best Efficiency Point.

Graphical explanation of the power consumption for identical pump speeds
Graphical explanation of the power consumption for identical pump speeds

And the Swedish dredge on 7 km of pipeline? It turned out, it was not a technical problem. They had no hurry. The contractor was hired per week on an open contract…

Auger dredge 'Detritus'
Auger dredge ‘Detritus’

References

  1. Welcome to Skanska, Skanska
  2. Södra Bergundasjön, Wikipedia (Swedish)
  3. Damen booster station, Damen

See also

Our Interview About New Pump Designs In The Latest Damen Nieuws

Headline of our interview in ‘Damen Nieuws’
Headline of our interview in ‘Damen Nieuws’

Another magazine dropped on my doormat, albeit a digital edition of ‘Damen Nieuws’1. The internal magazine for Damen colleagues. It featured an article with Suman Sapkota and me. Suman is our pump design specialist2 and at a Damen wide R&D convention he presented a poster on his pump design workflow within Damen Dredging Equipment. This caught the attention of the editorial board and we were interviewed on what we actually do for a living. Although we can’t share the exact details of the article or the poster, it is still an interesting message that we can highlight here.

Pump design workflow (inspired by Suman Sapkota)
Pump design workflow (inspired by Suman Sapkota)

The design of a dredge pump is based on the required specifications(1). The most important properties of the pump are: efficiency, NPSH, wear and ball passage. The first important property we try to fix is the ball passage3. We do use our own geometry generator(2) that assist us in creating a pump with a big ball passage. Unlike normal pumps, dredge pumps have to cope with debris and boulders that have to pass the impeller. The bigger chunks that can pass, the more uptime the dredge will have. Once we are satisfied with the geometry, we feed this through a file format converter(3). The resulting 3D file can be used on several platforms. This will enable us to create the digital solid for the engineering4, but it also gives us the negative volume, also known as fluid. Then to do mathematical operations on the digital fluid, we have to divide the volume into tiny cells. This process is called meshing.(4) When the mesh is available, the fluid flow through the mesh can be simulated with computational fluid dynamics.(5) All the fluid properties of every cell are calculated and the results are shared with the adjoining cells. This can be repeated until all properties of the cells don’t change very much anymore, a stable solution. Integrating all the properties of the cells give the resulting performance of the pump.

Balancing the four dredge pump performance properties
Balancing the four dredge pump performance properties

The estimated performance can be evaluated against the four properties.(6) The head times the capacity divided by the power required will give the efficiency. That is one of the items we wanted to know, as it relates to how much fuel will be consumed. The other parameter obtained from the CFD is the NPSH, or roughly: the suction performance. Wear cannot be estimated yet, but we are working on that2. Although the calculated turbulence might give a clue what wear to expect. If the properties are not satisfying our requirements we make an iteration in the geometry for improving the performance. However, changing the geometry will usually result in a smaller ball passage. If the parameters are OK to our requirements we have a pump design.(7) Manufacturing it is a completely different game.5

The design process of the dredge pump takes quite some effort and we are continually looking to improve the workflow6. Eventually we would like to be able to cater for all special requirements each individual customer might have.

Working for a dredge manufacturer, I am happy we design and produce our own pumps. It gives us the confidence, that when we supply dredges, they are as we like them to be. Another benefit is in discussions with the customer. It is easier when we can sit at the table as experts on their equipment assit them in finding a solution for their dredge.

Pump experts immersing themselves in checking the design of their pumps
Pump experts immersing themselves in checking the design of their pumps

References

  1. Damen Nieuws, Juni 2020, Damen
  2. Graduation Suman Sapkota: Where wear parts were worn down, Discover Dredging
  3. On The Relation Of Maximum Ball Passage And Recirculation Losses In Dredge Pumps, WODA
  4. Graduation Of Carsten Markus: Designing And Casting Of Impellers
  5. Don’t Play Games With Your Wear Part Planning
  6. Innovation, Damen

See also

Graduation Of Carsten Markus: Designing And Casting Of Impellers

Impeller under operating load.
Impeller under operating load.

Last week, Carsten Markus graduated on his assignment with our research department Damen Dredging Equipment. He investigated the alloys that are used to cast our impellers and the responses of the materials under operating conditions. We are always improving our dredge pumps and Carsten’s work has been a great contribution for our development.

For every dredging application, the material for the dredge pump parts can be carefully selected according the specific requirements on the sediment handled. Off course, one would like to have the hardest, most erosion resistant material available. Less erosion is less wear and a longer lifetime1.

Wear part material hardness in relation to wear index

However, there is a downside to choosing very hard material: it will be very brittle also2. Basically, the ultimate tensile stress coincides with the yield stress. There is no reserve for the load. When the stress surpasses the yield strength, it just snaps. Conversely, a tough material has a lot of reserve. After deformation beyond the yield stress, the load can still increase without a catastrophic failure. Usually, the stresses would not be that high, the thickness of the wear parts is dimensioned for erosion reserves and thick parts have low stresses. The high stresses can come from concentrations. Either when the wear reserves are eroded, or when a heavy load is concentrated on a very small area. Usually stones. Stones are a very common problem in a dredging project. So, next to the hardness of an alloy, also the toughness will be a very important characteristic. And toughness is related to the yield elongation after failure.

For the various materials you would like to know how the base load relates to the tensile stress. This would be an indication whether there is reserve in the elastic region to accommodate the impact of an occasional stone. This base load depends on many factors in the pumping process. Mainly the pressure generation in the mixture over the blades. Most known literature is about the force distribution in the volute of the pump, as this is directly related to the radial forces and consequently the bearing and shaft calculations. The CFD simulations in this graduation project revealed the skewed load on the volute and consequently the load variations on the impeller.

Transient CFD simulation of a dredge pump. The rotation appears wrong , but is correct (Wagon-wheel effect).

As emphasised before, it is very beneficial to operate your dredge system around the Best Efficiency Point (BEP). Not only the shaft and bearings suffer less, also the impact of the stresses and their variations in the impeller are less. If the operating point differs from the required capacity, the BEP can be moved by changing the dredge pump speed. As long as head requirements permit the adjustment.

Radial load as a function of pump characteristics.
Radial load as a function of pump characteristics.

As a result of Carsten’s research, we were able to improve our operating load models for the whole dredge pump and gained insight into the material responses to these. Thank you Carsten.

Due to the measures taken for the containment of the Corona virus, the graduation itself and the party to celebrate it, where done remotely over internet. Let’s enjoy the real beer later, after all this commotion is over. Stay home, stay safe.

Carsten’s graduation defence session under Covid-19 measures.
Carsten’s graduation defence session under Covid-19 measures.

References

  1. Do You Have Wear Parts For Spare?, Discover Dredging
  2. Brittleness, Wikipedia

See also